1. Field of the Invention
This invention relates to the field of automatic transmissions for motor vehicles. More particularly, the invention pertains to a kinematic arrangement of gearing, clutches, brakes, and the interconnections among them in a power transmission.
2. Description of the Prior Art
Traditionally, automatic transmissions have a planetary gearbox which provides a finite collection of selectable speed ratios and a torque converter which allows the engine to rotate even when the vehicle is stopped. The gearbox must provide a sufficiently low speed ratio to provide high output torque at low vehicle speed, a sufficiently high top gear ratio to minimize fuel consumption at highway speed, and enough speed ratios in between to enable comfortable shifts. The gearbox must also provide at least one negative speed ratio so that the vehicle can propel itself in reverse.
The primary function of the torque converter is to enable the transition from stationary to moving. A hydrodynamic torque converter transmits power from the engine to the gearbox input shaft whenever the engine speed exceeds the speed of the input shaft. When the gearbox input speed is zero, the torque applied to the gearbox is a multiple of the torque supplied by the engine. This torque ratio decreases as the vehicle accelerates and the speeds become more nearly equal. A substantial amount of the power supplied by the engine is dissipated by the torque converter whenever there is relative speed between the engine and the input shaft. Therefore, modern transmissions include a converter bypass clutch which is engaged at higher vehicle speeds to avoid this energy loss.
It is desirable to replace the torque converter with a launch clutch. This reduces fuel consumption in two ways. First, a torque converter will dissipate energy through slip until the bypass clutch is locked, whereas a launch clutch will be fully engaged at a low vehicle speed. Second, the clutch can be set at zero torque capacity when the vehicle is at rest, whereas a torque converter always places some load on the engine.
Launching a vehicle with a clutch presents several technical challenges. First, a slipping clutch does not multiply the torque as a torque converter does. Second, a clutch is not nearly as effective as a torque converter at dissipating heat. Both of these problems are most effectively addressed by providing lower gear ratios within the gearbox. This reduces the heat associated with a launch by reducing the input torque necessary to achieve a desired output torque and by reducing the time before the clutch is completely engaged. Finally, even when a clutch is disengaged, there is some parasitic drag. This drag tends to be higher for launch clutches than other clutches because the design is optimized for energy dissipation as opposed to drag reduction. If there is a high relative speed across a launch clutch in the higher gears, then this parasitic drag will have a large impact on fuel economy.
Although a lower gear ratio addresses some of the challenges, there are functional compromises associated with an extremely low gear ratio. When the vehicle is lightly loaded, it accelerates quickly. Once the launch is complete, the extreme gear ratio forces the engine to accelerate proportionately faster. The noise that results is considered unpleasant by many drivers and occupants. Also, the first shift occurs at a very low vehicle speed, which many drivers dislike.
Dual clutch transmissions are a class of automatic transmissions that use a blend of traditional automatic transmission components and manual transmission components. Essentially, a dual clutch transmission is a pair of automated manual transmissions; one for even gears and one for odd gears. Shifts between even and odd gears can be performed without torque interruption. Dual clutch transmissions do not typically have torque converters. At least one of the clutches, which can be either a wet or dry clutch, is used as the launch clutch and the advantages mentioned previously are realized. In addition to the functional compromises mentioned previously, these transmissions have other drawbacks relative to a planetary automatic. First, they typically use layshaft gearing, which is less efficient than planetary gearing for most ratios. Second, they are not able to perform two step shifts directly without torque interruption. For example, a shift from fifth to third would need to briefly engage fourth. This makes the vehicle seem less responsive to the driver.